Abstract
The efficiency of many automotive applications, for instance the ABS, could be increased by exploiting the absolute speed of the vehicle instead of that derived from the rotation of the wheels. Indeed, the later may not reflect the real situation, particularly in high danger cases, for instance when the wheels are sliding on an icy or wet road surface. Thus, sensors using Doppler effect are being investigated.
They take advantage of the frequency shift of a wave reflected by ground scattering obstacles. This shift, proportional to the vehicle speed, depends on the angle between the direction of the reflected wave and the road surface. Thus, the accuracy of the measurement depends on the knowledge of this angle. For this reason, conventional Doppler sensors use a narrow beam antenna in order to have a good accuracy on the angle. Such sensors are well suited to cases when the obstacle number is large. In some cases however, the probability of having scattering obstacles in the antenna footprint may be quite small, for instance when the road surface is icy, leading to the impossibility to perform the measurement. The present work was devoted to Doppler effect sensors using broad beam antennas in order to increase this probability. Two principles are proposed. The first one uses the simultaneous emission of two waves, a first one at a constant frequency and a second one which frequency is a function of time, whereas the second one uses the successive emission of these two waves. The measurements carried out with a laboratory set up and the results obtained by simulation techniques show that these sensors are particularly well suited for the above mentioned high danger situations when few obstacles are present on the road surface. In order to have a good accuracy for all road conditions, the directivity of the antenna can be electronically switched according to the obstacle distributions.